1. Field of the Invention
The present invention relates to an apparatus for recording a defective-position indicating signal at a defective position on a recording disk medium.
2. Description of the Prior Art
FIG. 5 shows the constitution of an optical pickup for a WO (write once) disk on which a signal is recordable only once.
In this diagram, reference numeral 1 denotes a laser diode (semiconductor laser) serving as a laser beam source, and reference numeral 2 denotes a laser driver for driving the laser diode 1. The laser beam outputted from the laser diode 1 is irradiated to a WO disk 7 through a coupling lens 3, a beam splitter 4, a 1/4 wavelength plate 5 and an objective lens 6.
In a recording mode, a record signal is modulated in a modulator 8 and is supplied to the laser driver 2, so that the power of the laser beam outputted from the laser diode 1 is modulated by the record signal (modulating signal), and then such record signal is recorded on the disk 7. Meanwhile in a playback mode, the power of the laser beam outputted from the laser diode 1 is kept at a fixed value.
The laser beam reflected by the beam splitter 4 is transmitted through a condenser lens 9 and is incident upon a photo diode 10 which serves as an optical detector. The detection output of the photo diode 10 is supplied to an APC (automatic power controller) 11, which then controls the laser driver 2 so that the power of the laser beam outputted from the laser diode 1 is controlled to an optimal value in each of the recording and playback modes.
The laser light reflected from the disk 7 is incident upon a dividing photo diode 14 via the 1/4 wavelength plate 5, the beam splitter 4, a condenser lens 12 and a knife edge prism 13. The divided output signals of the photo diode 14 are mutually subtracted in an operational amplifier 15 to be thereby formed into a focus error signal. This signal is then supplied to a focusing driver 16, so that the current flowing in a focus coil 17 is controlled with a focus servo action.
Meanwhile the laser beam reflected from the knife edge prism 13 is incident upon the dividing photo diode 18. The divided output signals of the photo diode 18 are mutually subtracted in an operational amplifier 19 to be thereby formed into a tracking error signal. This signal is then supplied to a tracking driver 20, so that the current flowing in a tracking coil 21 is controlled with a tracking servo action.
In a playback mode, the divided output signals of the photo diode 14 are mutually added in an operational amplifier 22 to become a playback signal, which is then supplied to a demodulator 23 to be thereby demodulated.
In a disk playback apparatus for reproducing an FM video signal recorded on a video disk, there occurs micro dropout (hereinafter referred to as "micro DO") peculiar to the video disk. Such micro DO is an individual dropout smaller than half the carrier wave and caused by a defect of the disk or dust deposited thereon. It is difficult to exactly distinguish between the video signal and the noise derived from such micro DO, and complete detection of the micro DO is not attainable. Consequently, there has been existent a problem relative to deterioration of the reproduced picture quality resulting from the micro DO.
FIG. 6 illustrates some kinds of dropouts, in which: (a) is a dropout of several .mu.sec; (b) is a half-wave dropout of the carrier; and (c) is a micro DO. FIG. 7 graphically shows the noises derived from such dropouts in a video signal.